1. Field of the Invention
This invention relates to a hot rolled steel wire rod or bar for machine structural use and a method to produce the same and, more specifically, to a steel wire rod or bar for machine structural use for manufacturing components for cars and construction machines and the like, enabling cold working such as drawing, machining, cold forging and the like by applying a short time spheroidizing annealing, and a method to produce the same.
2. Description of the Related Art
Machine structural components such as those of cars and construction machines and the like, for example, bolts, stabilizers, etc., have been manufactured conventionally by softening a hot rolled steel wire rod or bar of carbon steel for machine structural use or alloy steel to secure cold workability, then forming it by cold working such as cold forging, drawing, machining and the like, then quenching and tempering the pieces thus formed.
When manufacturing bolts from a hot rolled steel wire rod, for example, cold workability is secured by subjecting the material to one of the following alternative annealing processes for softening: a low temperature annealing for stud bolts and the like, which require light cold working; a common annealing for hexagonal bolts and the like; and a spheroidizing annealing for flanged bolts and the like, which require heavy cold working.
However, the time-consuming softening process, especially the spheroidizing annealing, which takes as long as about 20 hours, constitutes an obstacle to the improvement of productivity. Besides, the cost of the annealing has come to account for a considerable portion in the total manufacturing costs of the machine components and the like because of the recent rise in energy costs.
In this situation, from the standpoints of the improvement of productivity and energy saving, various technologies have been proposed to shorten the time of the spheroidizing annealing applied prior to the cold forming.
Japanese Unexamined Patent Publication No. S56-41325, for example, discloses a method to produce a soft wire rod not requiring softening in secondary working, wherein a hot rolled steel wire rod is subjected to a rapid cooling and then a controlled cooling under a specific condition to form a homogeneous fine pearlite structure in order that the wire rod is softened to an effective level. But the Publication does not disclose any technologies to obtain a wire rod having the same level of softness to withstand heavy cold working as is obtainable by the spheroidizing annealing.
Japanese Unexamined Patent Publication No. S60-21327, as another example, discloses a method to roll a steel wire rod at a first-stage hot finishing mill, rapidly cool it, impose plastic strain at a second-stage finishing mill and then cool it without removing the strain in order to facilitate spheroidizing at a subsequent process. But, this method is meant to accelerate the spheroidizing by means of the plastic strain and not by means of controlling the metallographic structure.
In view of the above situation, the object of the present invention is to provide, through the control of metallographic structure, a steel wire rod or bar for machine structural use having, even with a short spheroidizing annealing time, as good a cold workability as the steel wire rods or bars softened by the conventional time-consuming spheroidizing annealing, and a method to produce the same.
The present inventors directed attention to the structure of steel wire rods or bars obtained through the spheroidizing annealing process and studied a method to secure cold workability by achieving the spheroidizing and softening through a short-time spheroidizing annealing and obtaining a structure equivalent to that obtained through the conventional spheroidizing annealing.
The present inventor discovered that the steel wire rods or bars produced by hot rolling a billet having a specific chemical composition at a low temperature and cooling under a controlled condition had a novel fine ferrite-pearlite structure as shown in FIG. 1, wherein cementite in the pearlite is partially granulated, and that the high-temperature retention time of the spheroidizing annealing could be shortened to about one half of the conventional retention time by obtaining the above metallographic structure; and established the present invention on the basis of the finding.
The gist of the present invention, therefore, is as follows:
(1) A hot rolled steel wire rod or bar for machine structural use, characterized in that: the wire rod or bar is made from a steel consisting of, by weight,
0.1 to 0.5% of C,
0.01 to 0.5% of Si,
0.3 to 1.5% of Mn,
and the balance consisting of Fe and unavoidable impurities; its microstructure consists of ferrite and pearlite; its ferrite crystal grain size number defined under Japanese Industrial Standard (JIS) G 0552 is 11 or higher; the granular carbide 2 xcexcm or less in circle-equivalent diameter and having an aspect ratio of 3 or less accounts for a percentage area of 3 to 15%; and its hardness (Hv) satisfies the expression below,
165 Ceq+73.5xe2x89xa6Hvxe2x89xa6195 Ceq+73.5
(where, Ceq=C%+1/7 Si%+1/5 Mn%+1/9 Cr%).
(2) A hot rolled steel wire rod or bar for machine structural use according to the item (1), characterized by further containing, by weight, one or more of;
0.2 to 2.0% of Cr,
0.1 to 1.0% of Mo,
0.3 to 1.5% of Ni,
1.0% or less of Cu, and
0.005% or less of B.
(3) A hot rolled steel wire rod or bar for machine structural use according to the item (1) or (2), characterized by further containing, by weight, one or more of;
0.005 to 0.04% of Ti,
0.005 to 0.1% of Nb, and
0.03 to 0.3% of V.
(4) A method to produce a hot rolled steel wire rod or bar for machine structural use, characterized by subjecting a steel having the chemical composition specified in any one of the items (1) to (3) to a rough hot rolling in a temperature range from 850 to below 1,000xc2x0 C., a finish hot rolling in a temperature range from the Ar3 transformation temperature to 150xc2x0 C. above it, a controlled cooling through a temperature range from 700 to 400xc2x0 C. at a cooling rate of 5xc2x0 C./sec. or higher and, immediately thereafter, a temperature retention in a furnace atmosphere controlled in a temperature range of 500 to 700xc2x0 C. for 15 min. or longer but shorter than 1 h., so that the steel may have a ferrite crystal grain size number defined under JIS G 0552 of 11 or higher, contain the granular carbide 2 xcexcm or less in circle-equivalent diameter and having an aspect ratio of 3 or less accounting for a percentage area of 3 to 15%, and have a hardness (Hv) satisfying the expression below,
xe2x80x83165 Ceq+73.5xe2x89xa6Hvxe2x89xa6195 Ceq+73.5
(where, Ceq=C%+1/7 Si%+1/5 Mn%+1/9 Cr%).